This invention relates generally to ultrasonic scanners such as used for medical diagnostic purposes, and more particularly the invention relates to a method and means for providing improved picture display quality with dynamic weighting of pixel data in such a system.
Ultrasonic diagnostic systems are known and commercially available for diagnostic purposes. See for example U.S. Pat. No. 4,172,386 for "Video A Trace Display System for Ultrasonic Diagnostic System" and U.S. Pat. No. 4,204,433 for "Computerized Ultrasonic Scanner With Technique Select". The commercially available Datason ultrasound system of General Electric Company provides both real time and static images on a television display. The General Electric PASS and PASS II are real time imaging systems which use phased arrays of transducer elements.
Briefly, sush systems utilize sound transducer arrays to transmit ultrasonic (e.g. on the order of several megahertz) waves into a patient and to receive echo signals. The echo signals are applied to a time delaying focus circuit (element to element) with subsequent signal summation and logarithmic compresion and envelope detection before application to time gain compensated amplifier to adjust the echo signals for attenuation in passing through the patient. The adjusted signals are then passed through an analog to digital conversion and video processing circuitry and thence to scan converter circuitry for display formatting. The display comprises a plurality of pixels in horizontal rows ad vertical columns with each pixel having a brightness level in response to the input signal. Conventionally, the brightness is defined by a 2.sup.n level Gray-scale, hence the pixel brightness level requires an n bit digital code. The pixel brightness codes are stored in a random access memory with the memory periodically updated in response to video signals produced from the ultrasonic scanner.
In order to improve the signal to noise ratio of the displayed image, some weighted combination of old pixel data and new pixel data is commonly employed. With less random noise in theimage, echogenic targets of interest are better defined and therefore more clinically useful for diagnostic purposes.
Frame to frame video averaging is a common algorithm used to reduce the content or severity of random noise in the echo generated image in order to improve the perceived signal to noise ratio. Frame to frame video averaging, or temporal averaging, is a method by which the current displayed video data point (pixel) is combined with newly collected echo pixel value. The resultant pixel of that combination will be the next one displayed at that location.
The algorithm typically used is a fixed linear weighting average of those two pixels. For example, U.S. Pat. No. 4,375,671 for "Method and Means for Filtering and Updating Pixel Data" discloses the updating of pixel data by combining stored pixel data and data from the latest scan. Briefly, pixel data to be displayed is stored in a first random access memory. A second random access memory stores values of pixel data for updating the first memory. The second memory is addressed by using the present intensity value for a pixel and the intensity value for the pixel as stored in the first memory. The corresponding intensity value stored in the second memory is then used to update the first memory for the pixel.
A limitation in known pixel averaging techniques is the suppression of large or fast pixel grey level changes of short duration such as in imaging a valve in the heart. The excessive weighting of the currently displayed pixel data versus the new incoming pixel data introduces a hysteresis or persistence effect to moving targets in the two dimensional image. This persistence appears as a smearing or blurring of moving tissue as presented in the image display which is just as undesirable as the noise which the averaging algorithm is intended to suppress. This is particularly troublesome in cardiac imaging and diagnostic applications.
Accordingly, an object of the present invention is an improved method of updating pixel data without adversely affecting the imaging of moving tissue.
Another object of the invention is apparatus useful in the updating of moving tissue pixel data in cardiac diagnostic applications while suppressing random noise.
A feature of the invention is the dynamic weighting of new pixel data based on the differential between the new data and the stored pixel data.
The invention is based on the theory that the relative change in pixel amplitude, or contrast ratio, is proportional to target velocity or motion. The larger the change in pixel amplitude or Gray level, the faster the object being imaged is assumed to be moving. As the differential between new pixel data and old pixel data increases, the weighting of the new pixel data increases. Accordingly, fast moving targets will appear without persistence yet random noise at lower amplitudes will tend to be suppressed.
In a preferred embodiment of the invention the difference between incoming pixel data and stored pixel data is employed in a weighting factor for weighting the new pixel data and the stored pixel data in suitable lookup tables. The differential in pixel amplitudes along with the old and new pixel values are used to address lookup tables for dynamically weighted values for the new and old pixel values, and the dynamically weighted values are then combined to provide a new pixel value. A third address based upon the display dynamic range acts to normalize the effect of the selected dynamic range on pixel contrast ratio. Display dynamic range is the echo voltage range that will be mapped over the full range of digital pixel values by the ADC, and therefore relates to pixel contrast ratio.
The invention and objects and features thereof will be more readily apparent from the following detailed description and appended claims when taken with the drawing.